1. Field of the Invention
The present invention relates generally to lightweight composites and more specifically to lightweight composites with good acoustic damping properties.
2. Description of the Background Art
In many cases, such as in factories, motor vehicles, ships, and submarines, it is desirable to absorb as much noise as possible. In factories, absorbance of machinery noises can minimize noise pollution caused by industrial sounds. In motor vehicles, absorbance of engine noises and noises from road vibration enhances driver and rider comfort. In submarines and ships, the absorbance of machinery sounds helps prevent detection and identification by unfriendly forces. Additionally, in regions where earthquakes are common, the structural components of buildings, bridges, and similar artifacts must not only be strong, but should be able to dampen vibrations.
Past noise reduction efforts have previously taken one of two approaches. The use of soft coupling components (e.g., bushings, pads) to isolate machinery, and the incorporation of structural acoustic damping materials.
Both of these prior art approaches have had drawbacks. Soft coupling components add weight to a device, may require maintenance, and may not be consistent with the required design and performance features. Currently available acoustic damping materials do not exhibit adequate acoustic behavior over broad ranges of temperature, lack the required strain amplitude and frequency, or do not have the desired strength and environmental resistance.
Significant advances have been made to increase the strength and stiffness of lightweight materials. Examples of these developments include the superior density-normalized strength and stiffness of Al--Li, hard dispersion strengthened aluminum and titanium, aluminides, and whisker or fiber reinforced aluminum and magnesium. Although those materials have excellent structural characteristics, the demand continues for high strength, lightweight structural materials that also vastly improve damping capability.
U.S. Pat. No. 4,759,000, the entirety of which is incorporated herein by reference for all purposes, to Ronald P. Reitz describes acoustically transparent windows made of an aluminum/nickel foam impregnated with an acoustically non-absorbent rubber such as BE silicone rubber RTV-11. Nothing in that patent teaches or suggests that a useful material may be obtained by impregnating a metal foam with an acoustically absorptive polymer.
Additionally, demand exists for a lightweight structural material with high stiffness, isotropic mechanical strength, and outstanding fire resistance and high temperature capabilities.